So as to eliminate any disturbing elements introduced into the flow of a fluid by using traditional means for measuring the flow speed of the fluid, such as Pitot tubes or hot wire devices, laser beam velocimeters are preferably being increasingly used.
Devices for measuring the flow rate by a laser beam make it possible to, not merely avoid disturbing the flow, but also carry out measurements in normally inaccessible flow regions. For example, it is possible to measure through transparent cabin windows the flow rate of a fluid close to a wall of the pipe which carries the flow.
The velocimetric device generally comprise optical elements, an electric unit for processing the signal and a portion for computer-processing the measuring results. All these elements form a unit requiring a fine setting up and complex adjustments to draw up accurate measurements.
Current velocimetric device possess a certain number of adjustable parameters so as to correspond to various measurement applications.
As the estimates of uncertainties regarding the measurements of these devices provided by the manufacturer are often inadequate, it is necessary to calibrate and/or adjust the parameters of the device.
For devices operating on known flows, such as flows around an infinite containment cylinder or calculation modelizable flows, the adjustment and calibration of the velocimeter are obtained simply by comparing the experimental measuring results and the theoretical calculation.
On the other hand, so as to obtain extreme measurement accuracy for an unknown flow, it is essential to refer to a fully known flow simulation device so as to optimize the adjustments of the velocimeter.
Known calibration devices able to simulate a flow may generally be classed into two categories corresponding to the functioning of velocimeters.
The velocimeters of a first category emit two laser beams which interfere in a measuring zone. The passage of a particle of a fluid in a network of fringes of the interference region makes it possible to calculate the speed of this particle. This technique shall be expanded on in more detail in the continuation of the description.
For example, the simulation devices comprise a rough surface or surface with fine wires mounted on a rotating disk and pass into the interference region so as to simulate particles of the fluid.
The American patent U.S. Pat. No. 4,600,301 illustrates this calibrating device. The device described comprises wires stretched on a disk along beams, the disk being driven in rotation by a motor so as to make the wires reel off in the measuring zone.
This patent also refers to a device including a rough surface on a rotating disk simulating the passage of small particles in the measuring zone.
A second category of velocimeters emitting only a single beam function via the Doppler effect, that is by measuring the shift between the transmission frequency and the receiving frequency, this shift being caused by the speed of a particle, for example. As illustrated, in the American patent U.S. Pat. No. 4,176,950, the device comprises a velocimetric device emitting a single laser beam which traverses an optical path between the transmitter and a reflection mirror. Placed on the optical path is a block of a material transparent to the laser light and mounted on the shaft of a motor.
The rotating of the motor and the block causes a periodic and continuous modification of the optical path, thus making it possible to detect the receiving frequency and simulate a flow and able to be profitably used to calibrate the chain.
The devices for calibrating velocimeters of the first category functioning on an interference principle are unable to account for real flow conditions. In fact, the three-dimensional measuring volume defined by the intersection of the interfering beams is intercepted by a rough surface or wire extending inside a bidimensional plane. Thus, only one plane of the measuring volume is active. These devices are incompatible with certain "Doppler burst" detection methods (in which account is taken of an instantaneous multiplication of the number of events measured).
Furthermore, the device possesses a simulation element equipped with a rough surface resulting in a rapid saturation of the photomultipliers which are generally equipped with the velocimetric chains, as a rough surface, as it were, represents a continuous environment with a significant concentration of the measuring particles.
In this respect, the calibration devices of the second category are better adapted to the extent that they are freed from the physical simulation of particles.
At best, the simulation devices are able to account for flows with speeds satisfying a sinusoidal law. Moreover, the devices described earlier and comprising a transparent block rotating in the optical path of the laser beam are unable to simulate a flow other than a flow with a nil average speed.
Now, in order to accurately calibrate the velocimeter, it is essential to refer to a flow simulation as close as possible to a real flow. Real flows are generally characterized by an average non-nil speed and by a statistical distribution around the average speed. This is particularly the case with turbulent flows.
The aim of the present invention is to therefore provide a device for simulating and a device for calibrating velocimeters, said devices not possessing the drawbacks or limitations of the known devices described earlier but being able to calibrate the velocimeters with extremely high accuracy by simulating any real flow and in particular a turbulent flow.